1. Field of the Invention
The present invention relates to a laser-sensitive electrophotographic material. More particularly, the present invention relates to an electrophotographic material having an enhanced spectral sensitivity to semiconductor laser rays.
2. Description of the Related Art
Generally, a conventional zinc oxide-resin dispersion type electrophotographic material comprises an electroconductive substrate and a photosensitive layer formed on a surface of the substrate and comprising a principal component consisting of a finely divided photoconductive zinc oxide and an additional material consisting of a resinous binder and a sensitizing agent.
The zinc oxide contained in the photosensitive layer exhibits a photosensitivity only at a wave length of about 370 nm located in the ultraviolet band. Therefore, in the conventional electrophotographic material sensitive to visible light rays, the zinc oxide must be present in the photosensitive layer in combination with a sensitizing dye, to broaden the wave length range of light rays to which the photosensitive layer exhibits a satisfactory sensitivity.
Usually, the visible light rays generated from, for example, a halogen lamps, are used as a photographic light for the electrophotographic material. Due to the development of various recording machines such as laser printers and the spread of the digitalization of data, however, various laser rays, for example, argon laser rays, semiconductor laser rays, and helium-neon laser rays, are now widely used for the electrophotographic materials.
Among them, however, semiconductor laser rays, which have a large wave length of 700 to 1000 nm, are the most useful, since these semiconductor laser rays can be generated at a lower cost than that of other laser rays, and can be directly modulated and used in a smaller device than that needed for the other laser rays.
The conventional photosensitive layer containing the zinc oxide in combination with the sensitizing dye exhibits a very low or substantially no sensitivity to the semiconductor laser rays, and thus the conventional electrophotographic material is substantially useless when the semiconductor laser rays are used.
Various electrophotographic materials having an enhanced sensitivity to the semiconductor laser rays are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 57-46245, 58-58554, 58-59453, 59-22053 and 59-78358.
In those electrophotographic materials, the finely divided zinc oxide is contained in combination with a sensitizing dye, for example, a polymethine type cyanine dye, to extend the spectral wave length range of the usable light rays to which the electrophotographic materials are sensitive, to the long wave length side.
Nevertheless, this type of conventional electrophotographic material, in which zinc oxide is contained in combination with only the sensitizing dye, has an unsatisfactory sensitivity to the semiconductor laser rays. Especially, in recording machines, for example, a laser printer, the scanning exposure is carried out at a high speed, and thus the conventional electrophotographic material containing the sensitizing dye is not satisfactory or practical for semiconductor laser ray exposure.
Some of the conventional electrophotographic materials sensitive to the semiconductor laser rays contain, in addition to the sensitizing dye, a sensitizing assistant consisting of an electron-affinitive compound.
For example, Japanese Unexamined Patent Publication No. 1-16253 discloses a combination of a sensitizing coloring material consisting of a polymethine type cyanine dye compound having two terminal dimethyl indol ring structures each having an alkylsulfone radical attached to the N-substituent in the ring structure, with a sensitizing assistant consisting of maleic anhydride.
The above-mentioned type of electrophotographic photosensitive material has a high sensitivity sufficient for use for laser printers and laser plate maker, in which the semiconductor laser rays are utilized, but this conventional laser-sensitive electrophotographic material is disadvantageous in that the dark decay is large and the high humidity environment is increased.